The field of the invention is defrosting apparatus for refrigeration units and more particularly, those systems incorporating modified refrigerant flow as a means of defrosting.
Refrigeration units generally consist of an electrically driven compressor, a condenser, an evaporator and a flow restricting device. The compressor inlet is connected to draw refrigerant from the evaporator, and the compressor outlet is connected to discharge the refrigerant under increased pressure and temperature to the condenser. Under such conditions, the hot refrigerant entering the condenser is cooled by external means, usually air or water, thereby extracting heat from the refrigerant. As the temperature of the refrigerant drops under substantially constant pressure, the refrigerant in the condenser liquifies, or condenses, thereby losing additional heat due to latent heat of vaporization for the refrigerant.
The flow restricting device, usually either a capillary tube or expansion valve, is connected between the condenser and evaporator so as to maintain the high pressure in the condenser and at the compressor outlet while simultaneously providing substantially reduced pressure in the evaporator. The substantially reduced pressure in the evaporator results in a large temperature drop and subsequent absorption of heat by the evaporator.
The very cold temperature of the evaporator can result in formation of frost thereon, which eventually results in the buildup of ice on the surface of the evaporator. As the ice continues to collect on the evaporator, the transfer of heat is reduced due to the insulative effect of the ice. Further, the ice reduces the available space in the refrigerated compartment, which can be especially detrimental in small freezer compartments. Consequently, in order to remove the accumulated ice, it is necessary to periodically defrost the evaporator.
The most common defrosting method is the use of electrical resistance heaters, usually referred to as "strip" heaters due to their placement in elongated strips along door edges where frost tends to first accumulate. Other defrosting systems are known in which the flow of pressurized refrigerant out of the compressor is reversed, sending the hot refrigerant from the compressor into the evaporator rather than the condenser, in essence making the "evaporator" function as the "condenser", and vice versa. This is also the same principle as used in reversible "heat pump" systems for alternately providing heating and air conditioning. One problem with the traditional flow reversal method is the downtime necessary to reverse the flow of refrigerant to commence the defrost cycle, and then to again commence flow in the original direction. The flow reversal also requires a fairly complicated piping and valving system to accomplish.